1. Field of the Invention
The present invention relates to methods and apparatus for minimizing Imperfections In consolidated composite materials, and more particularly to a method and apparatus for eliminating fiber distortions ("fiber swimming") and separations ("fiber fish eyes") via a preliminary step taken before consolidation of "green", fiber-reinforced titanium matrix materials.
2. Background of the Invention
In the current methods of fabrication of titanium matrix composite (TMC) components, layers of woven fiber mat and thin foils (typically on the order of 0.005" thick titanium alloy) are positioned atop one another to form a "green pack" (see 110 in FIG. 1 of the drawings) which will later be consolidated by Hot Isostatic Pressure (HIP) in a steel tool at elevated temperatures (on the order of about 1520.degree. F.) and very high pressures.
To keep the foils and mats in place one atop another, an adhesive 120 is used to bond the foil and fiber mat layers. The "green pack" is placed in a recess of a stainless steel tool 130 and covered with a stainless steel sheet or "bladder" 140 that is welded to the tool. Stop-off compositions 150 often may also be used on the bladder surface facing the pack to help prevent sticking of the pack surfaces to the bladder. The volume between the tool and the bladder in which the pack is sealed from the atmosphere is commonly referred to as a "retort". The bladder is also designed to put positive pressure on the pack to prevent any internal movement during transportation and off-gassing. At a later time, during a "hipping" process, when a vacuum has been imposed in the retort, the bladder acts to apply pressure to the pack during its consolidation.
The off-gassing process involves subjecting the retort to elevated temperatures (on the order of about 850.degree. F.) while impressing a vacuum in the retort. As a controlled amount of inert gas is passed through the retort, a "purging" takes place in which the adhesive and stop-off compositions, which have volatized during the off-gassing operation, are driven off. These substances, if not removed, would contaminate the foils and fiber mat to prevent consolidation.
During the off-gassing process, the bladder is pulled against the pack by vacuum pressure. This has the effect of restricting fiber movement, breaking cross weave, and pushing the fibers in the mat apart, thereby causing undesirable "fiber swimming" (see the areas designated 170 in FIG. 2 characterized by a wavy or non-linear extent of the fibers) and "fish eyes" (see the areas designated 180 in FIG. 2 characterized by localized separations of adjacent fibers). The introduction of the purging gas into the retort was thought to be of some assistance insofar as the bladder would be pushed away from, and out of contact with, the pack, but this has not corrected the problem. The resulting anomalies degrade the overall quality of the composite as well as its mechanical properties.